The essential fatty acids (EFAs) are of two types, the n-3 or (.omega.-3) series derived from .alpha.-linolenic acid and the n-6 (or .omega.-6) series derived from linoleic acid. Linoleic acid and .alpha.-linolenic acid are like vitamins in that they cannot be manufactured in the body and therefore must be provided in the diet. The body can metabolise them along the pathways below and such metabolism is believed to be essential if they are to fulfil their functions.
The pathways are:
______________________________________ n-6 n-3 Structure ______________________________________ ##STR1## .DELTA..sup.9,12 octadecadienoic acid ##STR2## ##STR3## .DELTA..sup.6,9,12 and .DELTA..sup.9,12,15 octadecatrienoic acids .DELTA..sup.6,9,12,15 octadecatetraenoic acid ##STR4## ##STR5## .DELTA..sup.8,11,14 eicosatrienoic acid ##STR6## ##STR7## .DELTA..sup.5,8,11,14 and .DELTA..sup.8,11,14,17 eicosatetraenoic acids .DELTA..sup.5,8,11,14,17 eicosapentaenoic acid .DELTA..sup.7,10,13,16 docosatetraenoic acid .DELTA..sup.4,7,10,13,16 and .DELTA..sup.7,10,13,16,19 docosapentaenoic acid ##STR8## 22:6 .DELTA..sup.4,7,10,13,16,19 docosahexaenoic ______________________________________ acid
The acids are in the natural all-cis configurations. In the n-6-series commonly used names for the 18:2, 18:3, 20:3, 20:4 and 22:4 acids are linoleic acid, .gamma.-linolenic acid (GLA), dihomo-.gamma.-linolenic acid (DGLA), arachidonic acid (AA) and adrenic acid. In the n-3 series only .alpha.-linolenic acid (18:3) is commonly referred to by a non-systematic name.
The elongation stages in the natural metabolic pathway are much more rapid than the desaturations. The sequence believed to be mediated by common enzymes in the two pathways, are:
______________________________________ n-6 n-3 ______________________________________ 18:2 18:3 .DELTA..sup.6 desaturase 18:3 18:4 elongation 20:3 20:4 .DELTA..sup.5 desaturase 20:4 20:5 elongation 22:4 22:5 .DELTA..sup.4 desaturase 22:5 22:6 ______________________________________
The n-3 acids are metabolised preferentially and as a result plasma levels of 18:3 are low and 18:4 and 20:4 are in trace amounts only. In contrast the n-6 acids are all normally detectable, though 18:3 is at low levels, being apparently converted to 20:3 more rapidly than its relatively slow production from 18:2.